MKAbeFook for Facebook. Uses the WebKit browser with asynchronous requests, photo uploads, and much more. It can call any API method and returns the result in standard Cocoa types. It is distributed under a BSD style license.
Mashup games are always fun. Those built upon FourSquare have an extra layer of enjoyment, because they're tied to the real world. In the case of City Warfare, players use various water weaponry to soak other FourSquare users.